.back-top{
    position: fixed;
    text-align: center;
    top: 10px;
    width: 40px;
    height: 40px;
    margin-top: 10px;
    background-color: #424242;
    opacity: 0.7;
    border-radius: 5px;
    color:transparent;
    font-size: 11px;
    padding: 7px;
    z-index: 100;
    line-height: 1.2;
    visibility:visible;
}

.back-top-visible {
    visibility:visible;
}

.back-top-hidden {
    visibility:hidden;
}
